Looking good.
Yeah, Ghilliebear on the HKPRO forums can do it.
Its really not that tough to do yourself if you have a welder, the tough part is making sure that the trunnion is totally square with the rails before welding in.

Excuse my ignorance, but if someone does that type firearm work for another, isn't that consider manufacturing and subject to the proper license ??

From what I can gather, yes and no. Due to BATFEs gray area with regards manufacturing, owning the weapon and doing this (assembling or otherwise altering) for the purpose of selling it/profit would be considered manufacturing.

However, if you were to do this for a customer (have it shipped, do the work, and ship it back or otherwise take posession) it is gunsmithing, IF this is a registered 100% receiver, the customer owns it and is asking for the work to be done. General gunsmithing, if it requires transferring possession of the weapon (overnight posession) should be done by a licensed gunsmith with a federal firearms license and logged into the books.

If however the person doing the job can do it while you wait, the receiver is 100%, and no transfer takes place, the individual doing the work is neither manufacturing or taking possession, and consequently needs no license or federal permission.

I believe I have a pretty good understanding of the topic, but as is true in all dealings with the feds, do not quote me.